Ohio rest area showers are free, but don't forget to take your own towel! Also wanted to mention the trucker's lounge in Wendover, Nevada. They're free also, with towels, washcloth, and floor mat, plus a nice sized bottle of shower gel/shampoo!
They also have several casino and buffet perks for drivers, and a free shuttle to 3 of the casinos!
As far as tipping goes, if is well deserved, then yes I will leave a buck or two. So many times, you can tell that whoever cleaned the shower was in a big hurry, or just plain lazy, merely passing a rag over part of it. I went on one a while back at a Pilot, and there was urine on the toilet! Needless to say, I immediately asked for a different shower! Or, if there are a lot of drivers waiting in front of me, but the line moves fast and still the shower is clean, then I will leave a tip. Basically, it just depends on the circumstances!